Peripheral vision
' Peripheral vision' is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision. "Far peripheral" vision exists at the edges of the field of view, "mid-peripheral" vision exists in the middle of the field of view, and "near-peripheral", sometimes referred to as "para-central" vision, exists adjacent to the center of gaze. The loss of peripheral vision while retaining central vision is known as tunnel vision, and the loss of central vision while retaining peripheral vision is known as central scotoma. Peripheral vision is weaker in humans, compared with other animals, especially at distinguishing color and shape. This is because receptor cells on the retina are greater at the center and lowest at the edges (see visual system for an explanation of these concepts). In addition, there are two types of receptor cells, rod cells and cone cells; rod cells are unable to distinguish color and are predominant at the periphery, while cone cells are concentrated mostly in the center of the retina, the fovea. Flicker fusion threshold is higher for peripheral than foveal vision. Peripheral vision is good at detecting motion (a feature of rod cells). Peripheral vision is hard to study in an objective manner, because there is no way to separate the visual detection of the eye from the neural processing of the brain. While the eye can be dissected and examined under a microscope, even if the entirety of the retina is capable of detecting light, that capacity may not be fully utilized or may not be consciously aware within the brain. Certain conditions such as lazy eye can cause suppression of an otherwise usable visual field, while stroke or damage to the corpus callosum can prevent left/right integration. It is not possible to directly observe what the brain is detecting and comprehending, so research primarily involves perception tests based on reactions of test subjects to simulated stimuli. This testing is commonly carried out by requesting test subjects to focus on an object in front of them and then flashing lights at increasing distances away from the center of the visual field, noting the subject's reactions. Central vision is relatively weak at night or in the dark, when the lack of color cues and lighting makes cone cells far less useful. Rod cells, which are concentrated further away from the retina, operate better than cone cells in low light. This makes peripheral vision useful for seeing movement at night. In fact, pilots are taught to use peripheral vision to scan for aircraft at night. . Although peripheral vision is good at detecting motion (a feature of rod cells), however, and is relatively strong at night or in the dark, when the lack of color cues and lighting makes cone cells far less useful. This makes it useful for avoiding predators, who tend to hunt at night and may attack suddenly from ambush. The distinctions between foveal (sometimes also called central) and peripheral vision are reflected in subtle physiological and anatomical differences in the visual cortex. Different visual areas contribute to the processing of visual information coming from different parts of the visual field, and a complex of visual areas located along the banks of the interhemispheric fissure (a deep groove that separates the two brain hemispheres) has been linked to peripheral vision. It has been suggested that these areas are important for fast reactions to visual stimuli in the periphery, and monitoring body position relative to gravity. Peripheral vision can be practiced; for example, jugglers that regularly locate and catch objects in their peripheral vision have improved abilities. Jugglers focus on a defined point in mid-air, so almost all of the information necessary for successful catches is perceived in the near-peripheral region. Functions The main functions of peripheral vision are: * recognition of well-known structures and forms with no need to focus by the foveal line of sight. * identification of similar forms and movements (Gestalt psychology laws) * delivery of sensations which form the background of detailed visual perception. Peripheral vision loss may occur due to a number of ocular or neurological diseases or disorders. Glaucoma, stroke, branch retinal vein occlusion, branch retinal artery occlusion, ischemic optic neuropathy, and transient migraine are some of the more common causes, whereas retinitis pigmentosa, choroideremia, gyrate atrophy, pituitary tumors, optic disc drusen, brain tumors and aneurysms, and tilted optic discs are uncommon or rare http://www.eyemdlink.com/Condition.asp?ConditionID=342. Testing Peripheral vision # Close one eye. Extend your arms straight out to your sides # Slowly swing your arms to your front, noting when you first see your hands. # Do you see your hand opposite the closed eye hand 90 degrees from your nose? # Do you see your hand on the same side as the closed eye 30 degrees from your nose? # If yes, the test is normal. * IXMUS Color Field Test Peripheral Vision Testing for Macular Degeneration and Optic Nerve Disease See also * Averted vision * Eye movement * Eye movement in music reading * Fovea * Perimetry * Visual field * Vision span * Visual perception * Tunnel vision * Binocular vision References Category:Vision